The present invention comprises novel nanophase orientation methods for the preparation of composite high-Tc superconductors which contain a dispersion of nanophases and thereby exhibit enhanced flux pinning and enhanced critical current in the superconducting state. The present invention also comprises novel composite high-Tc superconductors prepared by such methods.
Superconductivity, ever since its discovery in 1911, has demonstrated enormous potential in industrial applications including nuclear magnetic resonance (NMR), magnetic levitation and propulsion, alternating and direct current power transmission, light weight generators, magnetic fission, high-field coils, and energy storage (Z. J. J. Stekly and E. Gregory, Chapter on Applications of A-15 S. C., xe2x80x9cIntermetallic Compounds, Principles and Practices,xe2x80x9d eds. J. H. Westbrook and R. L. Fleisher, J. Wiley and Sons, New York (1994)). All these applications have so far used the low-Tc superconductors (LTS) such as NbTi and NbSn, which are two commercially available compounds (D. Shi, xe2x80x9cProperties and Defects of Type II Superconductors,xe2x80x9d MRS Bulletin Vol. XVI, 37 (1991); Z. J. J. Stekly and E. Gregory, xe2x80x9cHigh Temperature Superconducting Materials Science and Engineering,xe2x80x9d (ed. D. Shi Pergamon, Oxford) p. 444, 1995). Although LTS materials offer unique properties, certain factors limit their usefulness in practice such as low Jc at high fields, brittleness, and an extremely low operating temperature at 4.2xc2x0 K. These factors have contributed to the fact that, despite the intensive efforts, superconductors have not become common in industry.
Theoretical and experimental research in the field of superconducting materials by thousands of researchers has led to the discovery of a variety of oxide compounds which become superconducting at relatively high critical temperatures (Tc), i.e., above about 20xc2x0K. The widely known high temperature superconductors are oxides, and presently contain (1) copper and/or bismuth, (2) barium or other alkaline earths such as strontium or calcium, and (3) trivalent elements such as yttrium. Rare earth elements having atomic numbers ranging from 57 to 71 (lanthanum to lutetium), are substituted for yttrium in some materials, as are thallium or bismuth.
In 1986, a series of high temperature superconductors (xe2x80x9cHTSxe2x80x9d) including YBa2Cu3Ox (123) and Bixe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O (BSCCO) were discovered with the Tc values exceeding 77 K (J. G. Bednorz and K. A. Mueller, Phys. Rev. B, 64 189 (1986); K. Wu et al., Phys. Rev. Lett. 58 908 (1987); H. Maeda, Y. Tanaka, M. Fukutorni and T. Asano, Jpn. J. Appl. Phys. 27 209 (1988)). These breakthroughs have indicated a promising future for superconductivity. With these HTS materials, it is possible to have applications at 77 K. Using liquid nitrogen as a coolant, the cryogenic systems can be greatly simplified making superconductivity application more realistic and economic. It has been found that not only the Tc""s of HTS are much higher, the upper critical field, Bc2, has been measured to reach a value on the order of 100 T (U. Welp, W. Kwok, G. W. Crabtree, K. Vandervoort, and J. Z. Liu, Phys. Rev. Lett., 62 1908 (1989)), making them ideal candidates for high-field applications. However, it has also been found that HTS materials carry extremely low critical current densities in the unoriented polycrystalline form as a result of their crystal anisotropy and grain boundary weak links (R. J. Cava, et al., Phys. Rev. Lett. 58, 1676 (1987)). The research effort has been focused on enhancing Jc by texturing grains and identifying coupling mechanisms at interfaces (S. E. Babcock, X. Y. Cai, D. L. Kaiser, and D. C. Larbalestier, Nature 347, 167 (1990)). These efforts have resulted in significant improvement in critical current density, particularly in YBa2Cu3Ox (123) (J. W. Ekin, Adv. Cer. Mater. 2, 586 (1987); D. Shi et al., Appl. Phys. Lett., 57 2606, (1990); K. Salama and D. F. Lee, Supercon. Sci. Technol. 7, 177 (1994)) and Bixe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O (BSCCO) systems (Q. Li, H. A. Hjuler and T. Freltoft, Physica C, 217 360 (1993); U. Balachandran, A. Iyer, P. Haldar, J. Hoehn, L. Motowidlo, G. Galinsid, Appl. Supercon. 2 251 (1994); H. Santhage, G. N. Riley Jr., and W. L. Carter, J. Metals, 43 21 (1991); R D. Ray II and E. E. Hellstrom, Physica C, 172 227 (1993)). For the 123 compound, Jc has reached the order of 104 A/cm2 at 5 T (P. G. McGinn et al. Appl. Phys. Lett. 57 1455 (990); R. L. Meng, C. Kinalidis and Y. Y. Sun, Nature 345 326 (1990); D. Shi, S. Sengupta, J. Luo, C. Varanasi, and P. J. McGinn, Physica C, 213 179 (1993)) indicating strong pinning strength in the system. But the current density in the BSCCO system is still limited, particularly at high fields above 30 K as a result of 2D vortex nature (K. E. Gray, Appl. Supercon., 2 295 (1994)).
Introduction of defects in intermetallic type II superconductors was proposed to increase their critical current density. See, for example, Campbell et al., xe2x80x9cPinning of Flux Vortices in Type II Superconductors,xe2x80x9d Phil. Mag., 18, 313 (1968).
However, in the case of high-temperature superconductors, the introduction of defects to increase critical current density to a useful level has met with only limited success. For example, in Gammel et al., Phys. Rev. Lett., 59, 2592 (1987), an increased density of twin boundaries provides only moderate improvement in flux pinning. Some increase in low temperature Jc in YBa2Cu3O7 in strong magnetic fields was achieved by the introduction of point defects by neutron irradiation in, for example, Willis et al., xe2x80x9cRadiation Damage in YBa2Cu3O7-x By Fast Neutronsxe2x80x9d, High Temperature Superintroductors, MRS Symposium Proceedings Vol. 99, 391-94 (1988). However, even in Willis et al., the increase in Jc was limited and at 70 K and B=4 T, increased to only about 104A/cm2 after about 1018n cmxe2x88x922 above which value superconductivity was adversely effected by the neutron dose. This may limit the wide application of neutron irradiation to provide improvement in flux pinning. Critical currents in polycrystalline high-temperature superconductors are still further reduced by weak links at the grain boundaries, which are made worse by high porosity, misalignment of the crystalline axis of adjacent grains, and by formation and accumulation of non-superconductor phases (compounds) at boundaries between superconducting grains.
The need for additional high temperature superconductors and methods of manufacturing superconductors is great, not only to achieve superconductors with higher Tc ""s, but also to achieve superconductors with improved Jc""s in magnetic fields, improved mechanical properties, stability, and ease of processing.
Previous findings show that the high-current applications of BSCCO are limited to low temperatures ( less than 30 K) since its Jc is rapidly reduced at 77K, particularly at appreciable applied fields. According to the previous experimental measurements, the Jc of Bi2Sr2Ca2Cu3Ox (Bi2223) tape is on the order of 104 A/cm2 at 77 K and self filed, but it is rapidly dropped to less than 1000 A/cm2 below 0.5 T for the field perpendicular to the surface of the tape. At 20 K for the same configuration, however, the decrease of Jc is within 10%, showing significant pinning strength. To increase the Jc in the higher temperature range, extensive effort has been devoted to the enhancement of flux pinning by manipulation of micro structure (D. Shi, M. Blank, M. Patel, D. Hinks, K. Vandervoort, and H. Claus Physica C 156, 822 (1988); K. C. Goretta, B. P. Brandel, M. T. Lanagan, J. G. Hu, D. J. Miller, S. Sengupta, J. C. Parker, M. N. Ali, and Nan Chen, IEEE Transactions on Apply. Superconductivity, 5, 1309 (1995)) and radiation damage (H. Safar et el., Appl. Phys. Lett. 67 130 (1995)). The former has been successful only at low temperatures and the latter is considered not practical for industrial applications. It is, therefore, important to develop new methods by which flux pinning can be sufficiently increased in the BSCCO system.
Various methods of increasing the Jc of superconductors are known in the art. These methods are generally though some type of xe2x80x9cpinningxe2x80x9d of flux lines through the introduction of defects into the superconductor material as described in the following patents.
U.S. Pat. No. 5,430,008, Morris, issued Jul. 4, 1995, discloses superconducting materials and methods of forming superconducting materials. Highly oxidized superconductors are heated at a relatively high temperature so as to release oxygen, which migrates out of the material, and form a non-superconducting phase which does not diffuse out of grains of the material. The material is then re-oxidized at a lower temperature, leaving the non-superconducting inclusions inside a superconducting phase. The non-superconducting inclusions act as pinning centers in the superconductor, increasing the critical current thereof.
U.S. Pat. No. 5,399,547, Negm et al., issued Mar. 21, 1995, provides a method for increasing the critical current density carried by high transition temperature superconductive materials. The methodology is employed using any Noble metal to form an electrically conductive coating, and is used with any high transition temperature superconductive material conventionally known.
U.S. Pat. No. 5,340,794, Tallon et al., issued Aug. 23, 1994, discloses a process of heat treatment to produce a high Tc superconductor with increased flux pinning. A precursor compound is subjected to temperature and oxygen pressure conditions at which the precursor compound decomposes or converts partially or completely to the high Tc superconductor and precipitated non-superconducting compounds which are dispersed through the structure of the high Tc superconductor and which are effective to pin lines of magnetic flux. The precursor compound may or may not itself be a high Tc superconductor. In the YBCO system, 2-4-7 may be converted to 1-2-3 or 1-2-3 to 2-4-7 and flux pinning copper oxides dispersed through the structure, for example, various other transformations are possible.
U.S. Pat. No. 5,308,800, Wehrle et al., issued May 3, 1994, discloses an apparatus and method for fabricating textured pseudo-single crystal bulk superconducting materials, featuring introduction of a pressure gradient and/or of a magnetic field for enhancement of known melt-texturing directional solidification techniques using a steep temperature gradient furnace for increasing temperatures at which superconductive properties can be maintained in superconducting materials. The strengths of the temperature gradient, pressure gradient and/or magnetic field can be optimally varied, resulting in formation of superconducting material exhibiting superconductive properties at higher temperatures than previously achievable.
U.S. Pat. No. 3,615,881, Greene, issued Oct. 26, 1971, relates in general to superconductive materials, and more particularly to vacuum-deposited superconductive films having improved characteristics. Type H superconductive material is bombarded with high velocity, heavy ions-which penetrate deeply into the material, causing disorientation in the lattice structure of sufficient magnitude to serve as flux pinning sites when the material is operated as a current carrier in a cryogenic environment below its critical temperature, and at field strengths between Hc1 and Hc2.
The present invention is targeted at the Jc enhancement in BSCCO at 77 K and respectable fields. A novel approach to the effective pinning of 2D vortices is provided. With this approach, Jc(H) can be significantly increased in BSCCO (both Bi2212 and Bi2223) at much higher temperatures ( greater than 40 K).